In some engines, diesel fuel has been used as a fuel efficient alternative to other fuels such as gasoline. In one example, air inducted into a combustion chamber of the engine is compressed by a piston and increased in temperature, while diesel fuel is injected directly into the combustion chamber to initiate combustion in the hot compressed gasses. This method forms a stratified mixture of air and diesel fuel, which when combusted may result in high production of NOx and soot, under some conditions. In another example, known as homogeneous charge compression ignition (HCCI), diesel fuel may be mixed with inducted air to form a substantially homogeneous mixture before being compressed to achieve auto-ignition of the air and fuel mixture. In some conditions, HCCI may produce less NOx and/or soot compared to stratified diesel combustion.
Under some conditions, it can be difficult to achieve a substantially homogeneous mixture with diesel fuel since it does not vaporize as readily as some other fuels such as gasoline. Furthermore, the timing of auto-ignition may be more difficult to control than stratified combustion where the injection of diesel fuel initiates combustion resulting in pre-ignition, knock or misfire. One approach used to improve the mixing of fuel, while controlling the timing of auto-ignition includes the addition of large quantities of exhaust gas recirculation (EGR). The EGR may be used to delay auto-ignition until a substantially homogeneous mixture is formed.
However, the inventors herein have also realized several disadvantages with the above approach. In particular, variations in EGR distribution between individual cylinders and/or engine cycles may result in auto-ignition of the mixture occurring too early or too late in the engine cycle. Furthermore, transient operation of the engine may exacerbate these variations, such as lag in EGR control that can result in uncertainties in combustion timing.
In one approach, at least some of the above issues may be addressed by a method of operating an internal combustion engine including at least a combustion chamber having a piston disposed therein, wherein the combustion chamber is configured to receive air, a first fuel and a second fuel to form a substantially homogeneous mixture, and wherein the piston is configured to compress said mixture so that auto-ignition of said mixture is achieved, the method comprising varying the amount of at least one of the first fuel and the second fuel that is received by the combustion chamber to adjust the timing of auto-ignition, where the first fuel includes diesel fuel.
In this manner, the combustion timing may be controlled by varying the ratio or relative amount of diesel fuel and a second lower cetane fuel utilized during each cycle. In some examples, the timing of combustion may be further controlled by adjusting the timing and/or quantity of these fuel injection(s). Thus, combustion timing control may be improved during transient engine operation and EGR usage may be reduced, thereby reducing engine pumping losses while increasing engine efficiency.
Furthermore, the inventors herein have also recognized that during cold ambient conditions, such as during engine start-up, it may be difficult to achieve HCCI operation with some fuel formulations. For example, it may be difficult to vaporize and/or ignite some low cetane fuels such as ethanol and methanol at low temperatures.
In another approach, the above issues may be addressed by a method of operating an internal combustion engine including at least a combustion chamber having piston disposed therein, wherein the combustion chamber is configured to receive a mixture of air and at least one of diesel fuel and a second fuel, the method comprising during a first condition, performing a first injection of the diesel fuel directly into the combustion chamber to form a stratified mixture of air and the diesel fuel, and to initiate combustion of the stratified mixture; and during a second condition, performing a first injection of the diesel fuel directly into the combustion chamber and a second injection of the second fuel into an air intake passage upstream of the combustion chamber to form a substantially homogeneous mixture of inducted air, diesel fuel, and the second fuel within the combustion chamber; and achieving auto-ignition of said substantially homogeneous mixture by compression ignition.
In this manner, fuel formulation can be adjusted in response to ambient conditions to improve engine start-up and warm-up operations while achieving the desired combustion timing.